The popularity of the game of golf has increased significantly over the past few decades, and the sport is now a major leisure time activity. Hitting a good golf shot (and thus achieving a good golf score) requires that the golfer know with some accuracy the distance the golfer needs to hit the golf ball. Off the tee, for example, the distance required in order to clear a fairway bunker or a water hazard is often a concern. When approaching the green, the distance to the pin or the distance to the front or back of the green must be known in order to enable proper club selection. In summary, then, golfers need to know where their ball lies relative to various points of interest on the golf course, including the pin, the front or back of the green, sand traps, trees, creeks, lakes, and other points of interest.
Various methods for determining the distance to golf course points of interest have been developed. These methods include techniques as simple as walking off the distance between the ball and the point of interest, referring to rough distance markers along the fairway (such as sprinkler heads or 150-yard-markers) and guessing based on vision. While these simple techniques have served golfers for centuries, other more technologically advanced systems have more recently been devised. More advanced systems include the use of binoculars with built-in laser range-finding devices and the use of electronic point identifier units. With the binocular-laser systems, the binoculars include a distance unit that enables a golfer to locate an object. When activated, the distance unit on the binoculars sends a laser signal to the object and the distance between the golfer and the object can be determined by evaluating the laser reflection. With the point identifier units, a stationary electronic unit may be placed on a point of interest, such as a flag pole, and when the golfer directs a point identifier unit towards the pin, the unit transmits a signal to the stationary electronic unit and determines the distance between the point identifier unit and the golfer unit. While each of these systems can be used to determine the distance to a golf pin or other point of interest, these systems have disadvantages. To overcome some of these disadvantages, range finders based on Global Positioning System ("GPS") receivers have been developed.
The United States Department of Defense developed and maintains a constellation of earth-orbiting satellites known as the Global Positioning System to provide radionavigation aids to commercial and military users. The satellite constellation consists of 24 satellites configured in six orbital planes. Radio transmissions from the satellites are referenced to very accurate atomic frequency standards aboard the satellites, which are, in turn, synchronized with a system-wide GPS system time base. The system provides accurate, continuous, worldwide, three-dimensional position and velocity information to users with appropriate receiving equipment. A number of standard GPS receivers have been developed for use in aircraft, boats, automobiles and other vehicles, and positioning receivers that use the GPS system have been widely deployed around the world.
The GPS satellites broadcast a pseudo-random-noise code ("PRN code") and navigation data modulated on two carrier frequencies via a spread-spectrum technique called Code Division Multiple Access ("CDMA"). All satellites transmit signals with the same two fequencies, but each satellite uses a PRN code that is relatively orthogonal, and hence uncorrelated, with respect to the CDMA codes used by other satellites in the constellation. Navigation data transmitted by each satellite allows receivers to determine the location of the transmitting satellite at the time of transmission, while the PRN code allows receivers to determine which satellite transmitted the signal. The transit time of each signal, and hence the satellite-to-receiver range and the position of the receiver, can be determined once the signals from multiple satellites have been distinguished.
GPS provides service enabling positioning accuracy of about 100 meters to any user worldwide. However, U.S. military users can access the GPS "precision" service that provides considerably more accuracy if they have keys to an encrypted dithering of the satellite clock. In addition, all users can use "differential" techniques to obtain far greater position accuracy, such as by removing correlated errors between two or more receivers when the location of one receiver is very precisely known beforehand.
The most common method of calculating position using the GPS satellites is to measure the receiver's distance (or "range") from at least four GPS satellites in known locations, and to triangulate the receiver's position therefrom. The range from the receiver to each satellite is measured indirectly by measuring the time for the radio signal transmitted by each satellite to travel through space to the receiver.
This time measurement is made through the use of the PRN code generated by each satellite and transmitted at a precisely determined interval of GPS time that is common to all the satellites. Standard GPS receivers are equipped to generate internally an exact duplicate of the unique code transmitted by each satellite. The receiver "tracks" or "locks on" to a particular satellite by matching or correlating the internally generated code assigned to a particular satellite to the code received as part of the signal transmitted by that satellite. Since some amount of time is required for the coded signal to travel at the speed of light from the satellite to the receiver, the receiver must delay the generation of the internally generated code in order to match the internally generated code to the code being received. By measuring the amount of time by which the internally generated code must be delayed in order to match the coded signal received from the satellite, the receiver is indirectly measuring the time required for the signal to travel from the satellite to the receiver. Multiplying the time required to travel from the satellite to the receiver by the speed at which the radio signal is traveling (the speed of light), the user can determine the "range" or distance to the satellite.
A GPS receiver at a known (i.e., surveyed) location can be used to provide correlated error corrections which can then be transmitted to other nearby receivers to enable more accurate position determination. These receivers use these error corrections in combination with their GPS-derived apparent position data to produce a more accurate differential GPS (DGPS) position correction. DGPS techniques are frequently implemented to achieve a higher degree of accuracy than possible with absolute (single receiver) measurements. When both the base station units and mobile units are within a few miles of each other, DGPS can remove common-mode errors that affect absolute single receiver measurements. These common-mode errors include selective availability (SA) and bias errors, such as satellite clock errors, ephemeris data errors and tropospheric delay effects. DGPS does not correct errors due to multi-path or noise detected at the receiver.
In a typical DGPS-based golf range-finding system, the location of a golfer or mobile unit associated with the golfer may be determined using a standard GPS receiver. Many of these range-finding systems include a fixed, central base station at the golf course clubhouse and numerous mobile units that are either mounted on golf carts or carried by the golfer. Mobile units typically include both a GPS receiver for position determination and a radio communications transceiver for communicating with the central base station. The mobile units typically store a database of records that include the locations of various points of interest on the golf course. The base station unit calculates its GPS position from the signals and compares the calculated position signal to the known fixed location of the base station to compute a differential position correction. These differential position corrections are transmitted to the mobile units to enable the mobile units to correct for correlated positioning errors and to thereby determine a more accurate position estimate than would otherwise be possible with an uncorrected GPS position reading.
While using GPS- and DGPS-based golf ranging systems to provide position estimates to conventional mobile distance units operated by golfers is useful, pin changes and the associated updates must be completed before the golfer begins the round so that the mobile unit can be updated with the correct positions. Because the pin positions on a golf course are routinely changed in the morning after some golfers begin to play, the mobile units operated by the golfers may not have the most recent pin positions information. In other GPS mobile distance systems for golf courses, the pin positions are determined by basing distance calculations on a pre-determined daily pin area position. With the day of week identified, the distance to the pin position area for the day of the week can be determined. However, these estimates may not be as accurate as a golfer would like. Furthermore, although DGPS systems are accurate, some golfers, particularly expert golfers, prefer that the system yield a better margin of error than these systems typically provide.
In order to provide a more accurate GPS-based position determining system for critical aircraft traffic control and automated landing systems, the U.S. Federal Aviation Administration is developing the Wide Area Augmentation System (WAAS). The WAAS improves the accuracy capability of a GPS-based positioning system in much the same way that a ground-based DGPS system does. WAAS is based on a network of approximately 25 ground reference stations that covers a very large service area in North America. Signals from GPS satellites are received by wide area ground reference stations (WRSs). Each of these precisely surveyed reference stations receives GPS signals and determines a correction signal required to correct for errors in the GPS-derived position at that point. These WRSs are linked to form the U.S. WAAS network. Each WRS in the network relays the data to the wide area master station (WMS) where correction information is computed. The WMS calculates correction algorithms and assesses the integrity of the system. A correction message is prepared and uplinked to a geostationary communications satellite. The WAAS correction message is then broadcast by the geostationary satellite at the same frequency that the GPS satellites transmit the GPS navigation code (1575.42 MHz) to WAAS-equipped receivers onboard aircraft or on the ground that are within the broadcast coverage area of the WAAS. Because the WAAS signal is broadcast at the GPS navigation signal frequency, standard GPS receivers can be easily modified to receive and decode the WAAS correction message and to thereby enable more accurate position determination than is possible using GPS alone. It is expected that in the near future many GPS receivers will be equipped with built-in WAAS capability.
Conventional GPS-based golf range-finding systems suffer from a number of practical limitations, unfortunately Golf courses increasingly require that golfers refrain from driving their golf carts into the fairway and that the carts instead remain on the asphalt or concrete "cart paths" that extend from tee to green on one side of a typical golf hole. Such "cart path only" rules help maintain the quality of the turf in the fairway, particularly after a rain when the ground is soft. GPS-based range-finding systems that are mounted in the cart are thus of little use if the cart cannot be driven to the golfer's all. Portable GPS-based range-finders that can be carried by the golfer to the ball have been developed to solve the "cart path only" problem and to service golfers who walk the course rather than ride in a cart. In order to take advantage of real-time DGPS or WAAS position correction systems, communications links to a central base station, and the availability of large display screens for displaying course detail and other rich information, however, these portable units typically must include an RF transceiver, significant battery storage, and a large display screen in addition to a GPS receiver and distance determination capability and are thus large, heavy and more expensive.
Unfortunately, conventional GPS-based golf range-finding systems thus require a performance and cost trade-off. If the system is cart-mounted, full features, a large display screen, and rich functionality can be provided, but the system is not useful in "cart path only" situations or when the golfer otherwise leaves the golf cart. If, on the other hand, a portable unit is desired, features and functionality must conventionally be sacrificed in order to minimize size, weight, power consumption, and cost.